The separation of mixed component feeds into a first fraction rich in one or more components of the feed and a second fraction lean in those same components has been long pursued by industry. Gross separations have been achieved by such procedures as distillation, solvent extraction, extractive distillation and, most recently, membrane separation. In each instance, the degree of separation achieved and the purity of the product recovered has risen as the sophistication of the process employed has improved and so long as the components separated possessed characteristic which facilitate their separation. Thus, while when the components of a mixed component feed possessed substantially similar boiling points distillation would be an inadequate separation procedure, separation could still be achieved if the components differed in terms of aromaticity, polarity or molecular size, etc. Such differences would make the feed amenable to separation by extraction, size dependent membrane separation (e.g., ultrafiltration or reverse osmosis) or solubility dependent membrane separation (e.g. perstraction or pervaporation).
Residual oils contain substantial quantities of molecules that are suitable for cat cracking; and separation of those components by various means has been the target of research for a number of years. The major focus has been in the reduction of metals and Con carbon in the fraction going to the cat cracker to minimize the effect of these species on catalyst activity and life.
Clearly, one way to accomplish a rough separation of this type is deasphalting using propane or mixed propane/butanes; however, this separation is not as clean as required and every refinery does not have a deasphalter.
Ultrafiltration using polymeric or ceramic membranes and reverse osmosis (both pressure-driven processes) have also been evaluated in recent years but to date have not been able to achieve the degree of separation needed to make the process attractive. Furthermore, it has been found that in ultrafiltration it is not the membrane that is actually doing the separation but rather a gel layer of larger molecules that forms on the surface of the membrane after a short time. This gel layer decreases the flux and is not very selective.
Concentration-driven membrane separation processes (pervaporation and perstraction) have been investigated in recent years for the separation of aromatic from non-aromatic species; and some testing has been done on heavier lube stocks. Results show clearly that diffusivity through the membrane decreases markedly (as does volatility) as molecular weight increases. Therefore, pervaporation is not feasible for residual oils and perstraction using conventional solvents also does not work very well for even heavier lube stocks.
Perstraction is a concentration-driven membrane process in which a liquid feed is contacted with one side of a non-porous membrane, whereupon a portion of the feed selectively dissolves into and then diffuses across the membrane. Removal of the permeate molecules from the downstream side of the membrane is accomplished by sweeping the membrane with a fluid that does not contain the permeating species. Therefore the concentration gradient that drives diffusion is maximized.
The importance of perstraction will arise not so much from the separation of light feeds (for which separation pervaporation can easily be used) but in the separation of heavier feeds--for example, the separation of aromatics from non-aromatics in jet fuel or diesel fuel or the separation of aromatics from coker naphthas and gas oils which contain diolefins which will oligimerize at higher pervaporation temperatures forming harmful gums, or the separation of non-asphaltenes (also called maltenes in the literature) from asphaltenes.
The use of solvents at condition at or above their critical temperature and pressure in selective extraction procedures is also well known and the subject of numerous patents, see: U.S. Pat. Nos. 2,980,602; 2,940,920; 4,125,459; 4,278,529; 4,239,616; 4,273,644.